This invention relates generally to an improved fired heater for a coal liquefaction process. More particularly, this invention relates to a method of operating a fired heater for a coal liquefaction process so as to improve the heat transfer efficiency thereof.
In the conversion of coal to synthetic fuels by direct liquefaction, the coal is mixed with a recycle solvent and is hydrogenated in a three phase reactor at temperatures in the range of 750.degree.-880.degree. F. (399.degree.-471.degree. C.) and pressures in the range of 1000-3000 psi (6.89.times.10.sup.7 -2.07.times.10.sup.8 dynes/cm.sup.2). In a direct coal liquefaction process, for example the SRC-I process, coal is mixed with solvent at low temperature (typically from 100.degree.-450.degree. F.) (38.degree.-232.degree. C.) at atmospheric pressure. The resulting slurry is pumped to a high pressure, for example, 2500 psi (1.72.times.10.sup.8 dynes/cm.sup.2) and is then preheated in heat exchangers to a temperature of approximately 500.degree. F. (260.degree. C.) Hydrogen gas is then added to form a three phase mixture of hydrogen/coal solvent which is heated to a temperature of 650.degree.-800.degree. F. (343.degree.-427.degree. C.) in a fired heater by passing the mixture through a pipe having a very long length to diameter ratio. The preheated three phase mixture is then passed to a reactor vessel in accordance with the SRC-I process.
The fired heater is a critical component in a process for the direct liquefaction of coal. Because of the high operating pressure and temperature and the erosive/corrosive nature of the coal slurry, expensive materials are required for the fired heater making this unit a major cost item in the coal liquefaction process.
As stated above, the function of the fired heater is to heat the hydrogen/coal/solvent three-phase mixture flowing from the slurry preparation stage to the dissolver. The fuel required to heat the feed to reaction temperature is a major expense in any coal processing plant. In order to minimize the total energy load required for preheating and thereby reduce the fuel requirements of a plant, heat exchangers may be injected into the feed system to raise the temperature to as high a level as possible by using heat generated from other areas of the plant from various cooling steps. Heat transfer media or suitable substitutes are commonly used to effect such heat transfer from one location to another. However, it is still necessary that considerable heat be added to even a pre-warmed slurry to get it up to reaction temperatures.
In coal liquefaction plants the efficiencies in the fired heater both in equipment and fuel requirements can have a major impact on the cost of building and operating such a plant. Since the process operates at high pressures, very expensive equipment is required to contain the very corrosive reaction media. The major cost associated with the fired heaters based upon heaters having nearly the same level of heat input is the length and size of the fired heater tube. Generally, the shorter the tube length at constant heat input the less expensive the total fired heater. Alternatively, for the same tube length, the heating rate to the fired box can be turned down to reduce fuel expense. The behavior of three-phase mixtures flowing through such systems at high heat fluxes also constrains the size and shape of these tubular systems. On the one hand, utilizing a system with a large tubular diameter will result in diminished heat transfer to the reaction media. On the other hand, using a diameter which is quite small will result in problems associated with very high erosion rates within the tubes brought on by the very rapid movement of the slurry through the pipe.
Fired heaters can be of several configurations. The pipes can run in horizontal or near-horizontal configurations slowly spiraling upward as the pipe winds its way around a circular or race track type pathway. Because of the long lengths of pipe often used, the height of such units becomes quite large, and because of the costs associated with erecting high structures, a cost incentive exists to minimize the overall height of these structures.
Another configuration used in these fired heaters is an up and down pattern resembling an upright radiator and comprised of a series of hairpin turns at the top and bottom. Because of problems associated with materials that could accumulate in the lower bends such as design is less favorable for use in a coal liquefaction plant.
It is known in the art of coal liquefaction to utilize the horizontal or near-horizontal design. Likewise the art has comtemplated that operation of a preheater system need not be restrained over the overall operating range that a plant may experience. In U.S. Pat. No. 3,884,794 a coal liquefaction process is disclosed in which hydrogen may be passed to the preheater at a rate from 200 to 10000 SCF/Barrel (35.6-1779.9 scm/m.sup.3) of feed. This corresponds to approximately 1300 to 65000 SCF/ton (4.06.times.10.sup.-2 -2.03 scm/Kg) of coal at 40% coal slurry concentration. The overall operation of the preheater is contemplated to occur at ratios of gas to slurry through the preheater of 0.2/1 to 13/1.
The inventors have found that when a gas-slurry system flows through a horizontal or near-horizontal pipe at gas superficial velocities from 1 to 20 ft/sec (0.30-6.10 m/sec) and slurry superficial velocities greater than 1 ft/sec (0.30 m/sec), slug flow occurs. Slug flow refers to a behavior of the mobile phase in the pipe wherein the slurry phase will intermittently bridge the cross-sectional area of the pipe. Most of the time the top section of the pipe will be in contact with "slugs" of gas which are moving through the system. Heating the contents of the pipe would be far more efficient if the slugs of gas could be eliminated thereby allowing the slurry to completely fill the pipe bridging the cross-sectional area as it progresses through the preheater from one end to the other. Such a mode of operation puts slurry in contact with the walls most of the time thereby increasing heat transfer.
Unfortunately, in coal liquefaction preheaters in which three-phase flow must occur such completely flooded pipe designs are not acceptable. Froth flow would accomplish such a desired uniform flooding effect, but such behavior can be accomplished only at very high velocities where erosion by the particulate material would be severely limiting. It is known to those skilled in the art of heat transfer and fluid mechanics that higher heat transfer will also occur at higher frequencies for systems operating in a slug flow mode. These higher slug frequencies are also accompanied by higher slurry linear velocities through the tube which means that at higher slurry rates through the pipe heat transfer per surface area of the pipe will be more efficient.
Under all circumstances some hydrogen must be in contact with the flowing slurry in order to retard the coking that often happens in its absence. Therefore, since some gaseous hydrogen must be present, it is desirable to minimize the gaseous phase to as low a level as possible for acceptable operation in order to maximize heat transfer by maximizing contact of the slurry with the total pipe wall surface. Based on the fact that some hydrogen gas must be present, higher heat transfer is also helped by maintaining high slug frequencies in the pipe. This promotes frequent complete bridging of the pipe diameter.